In recent years, the need for global data networking capability has rapidly expanded. Computers at one location often have a need to transmit data to computers at a distant location. In order to meet this need, broadband satellite communication systems have been proposed as an alternative to land-based communication systems. One type of satellite communication system is described in a number of U.S. patents assigned to the assignee of this patent application, including U.S. Pat. Nos. 5,386,953; 5,408,237; 5,527,001; 5,548,294; 5,621,415; 5,641,135; 5,642,122; 5,650,788; 5,736,959; and 5,740,164. These patents and other pending applications assigned to the assignee of this patent application describe a satellite communication system that includes a constellation of low-Earth-orbit (LEO) satellites capable of transmitting data from one location on the Earth's surface to another location. Each LEO satellite has a communication footprint that covers a portion of the Earth's surface as the satellite passes over the Earth. The communication footprint defines the area of the Earth within which ground terminals can communicate with the satellite. During the period of time a ground terminal remains within the borders of a satellite footprint, the ground terminal may transmit data to and receive data from the “servicing” satellite. When a satellite reaches the end of its servicing arc, another satellite in orbit is positioned to “service” the ground terminal previously covered by the satellite reaching the end of its servicing arc.
Data to be sent from one location on the Earth to another location is transmitted from a source ground terminal to the satellite servicing the ground terminal via an uplink data channel. The data is routed through the constellation of LEO satellites to the satellite servicing the ground terminal that is to receive the data. The latter satellite transmits the data to the destination ground terminal via a downlink data channel. Thus, the constellation of LEO satellites and the ground terminals form a data communication network wherein the ground terminals and satellites form nodes of the network.
A data communication network uses a transmission protocol to govern the format and manner in which data communication takes place in the network. Data to be communicated through a network is typically broken up and formatted into digital data “packets.” The resulting data packets are addressed and routed through the network to a destination terminal in accordance with the transmission protocol used by the network.
In a preferred embodiment of the LEO satellite communication system described above, the data packets are routed independently through the satellites in the constellation. The data packets may therefore arrive at the destination ground terminal in a different order than which they were sent by the source ground terminal. The communication handoff from one servicing satellite to the next servicing satellite and changes in the phasing of satellites in their orbital planes may also contribute to out-of-order delivery of the data packets.
For circumstances in which in-order packet delivery is required (e.g., by the transmission protocol of the data packets), sequence information may be included in the data packets prior to transmission. While sequence information that is included in a data packet may be used to reorder the transmitted data packets, the sequence information adds to the overhead information for routing and control that is transmitted in each data packet. Where transmission resources in a data communication network are limited, adding to the overhead information in transmitted data packets reduces the communication efficiency of the data communication network. The added information uses resources that could otherwise be used for transmitting user data. A need therefore exists for a method to minimize the number of bits used in a data packet for sequence information so that transmission overhead is reduced while still providing a mechanism for reordering packets delivered out of order.
Furthermore, a destination terminal may receive data packets from a number of different source terminals. Without using dedicated communication links to effectively identify the source of the data packets and the particular data transmission in which the packets were sent, the data packets themselves must include source information so that the destination terminal will know how to organize and process the incoming data packets. In some existing networks (e.g., networks using Internet Protocol), data packets include a source terminal's complete network address in their routing and control information to identify the transmission source. However, as noted above, adding to the transmission overhead information reduces the data communication efficiency of the network. A complete network address is often longer than necessary to adequately identify a data transmission source to a destination terminal. A need therefore exists for a method to minimize the number of bits in a data packet allocated for a source identifier.
A need also exists for a method of setting up a source identifier having a minimal number of bits allocated to the source identifier, as discussed above. In some existing networks (e.g., networks using Frame Relay or Asynchronous Transfer Mode (ATM) protocol), contexts are set up for data transmissions from one point to another in the network in which labels are set up to identify the transmission over a particular link. However, these labels have no meaning outside of that particular point-to-point link. In other networks, header compression schemes have been used in which routing and control information in a data packet is compressed and later reconstructed. These compression schemes, however, have been limited to point-to-point links as well. Accordingly, there is a need for a method of data transmission that spans a plurality of point-to-point transmission links and uses a reduced-size source identifier that has meaning to the destination end of the data transmission.
As noted above, a transmission protocol governs the format and manner in which data transmission takes place in a data communication network. Those skilled in the art of data communication will appreciate that different computer networks may use different transmission protocols that are not directly compatible with one another. For example, an Internet Protocol (IP) network may incorporate subnetworks and links that use ATM protocol. An adaptation is required before an IP packet can be communicated through an ATM link. In order for a LEO satellite data communication network to be competitive in a market with diverse end users desiring to transmit data via the satellite network, the satellite network must allow for transmission of data packets formatted according to a wide variety of data transmission protocols that are potentially incompatible with the transmission protocol of the satellite network.
Moreover, because end users typically use a satellite network only as an intermediary in the transmission of data, it is preferable that the operation of the satellite network be “invisible” to the end users. Accordingly, there is a need for a data transmission method that efficiently transmits different protocol data packets through a satellite network such that the data packets received by an end user at the receiving end of a data communication are in the same format as sent by an end user at the sending end of the communication, even when the packets are received out of order.
The present invention provides a method of data transmission that addresses the above-identified needs and other shortcomings in the prior art.